Frequency modulated wave receiver



FREQUENCY MODLATED WA'YE RECEIVER Feb. 17, 19452.

D. E. FOSTER 1an-1m. FREQUENCr MODULATED WAVE RECEIVER 3 Sheets-Sheet 2 Y Filed- Feb. 20, 1940 SRSQ m 5 Nm 5R WG un K n AM nu. R7 W M A/u.

000 /NPU T T0 ANTENNA F; M. DETECTOR CHA/2A cTER/s-T/c means/vcr nv Mc f -0 l Calw/20L TUBE /As VOLTS v zNvENroRs DUDLEY E. FOSTER AND fm A. RANK/N ATTORNEY.

Feb. 17, 1942. D; E; FOSTER ETAL 2,273,097

, FREQUENCY MODULATED WAVE RECEIVER Filed Feb. 20, 1940 5 SheetS-Sheet 3 I@ 5 A ML/f /SGNAL s/GNAL 0N 6R10 w Zi/7g@ 5 5 our/ur SIGNAL .SVGA/AL o /v G12/D' INVEN T ORS DUDLEY E.. FOSTER AND ATTORNEY.

Patented Feb. 17, 1942 UN1TED STATES. PATENT -oEFlcE FREQUENCY MoDULA'rEn WAVE EEcErvEE Dudley E. Foster, South Orange, N. J., and John A. Rankin,'Jackson Heights, N. Y., assignors to RadiDelao Corporation of -America, a corporation of Application February zo, 1940, serial No. 319,830

11 claims. (ci. 25o- 20) Our present invention relates to radio receivers of the ultra-high frequency type, and more particularly to a system adapted to receive frequency modulated waves.

In co-pending application Serial No. 320,103, filed February'21, 1940, by D. E. Foster there is disclosed a frequency modulated wave (hereinafter referred to briefly as FM) receiver adapted .to receive signal waves located in the ultraistic, Fig. 4 graphically shows the frequency control across a time constant network whose discharge time constant is not too large to follow amplitude iiuctuations of the highest frequency it is desired to limit.

Another-important object of this invention is to provide in an FM receiver an improveddetector of the back-to-back type consisting of two high frequency signal receivers, and more particularly to provide an FM receiver capable of being economically manufactured and easily ase sembled.

The novel features which we believe to be characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

In the drawings:

Fig. 1 shows a circuit diagram of an FM re ceiver embodying our invention,

Fig. 2 graphically shows the limiter characteristics,

Fig. 3 illustrates the FM detector charactercharacteristic of the AFC tube.

Fig. 5A illustrates the e'ect on the limiter action of a discharge time constant which is too l Fig. 5B shows the improvement secured in the limiter action when employing our invention.

Referring now to the accompanying drawings, and specically to Fig. `1, the networks of the FM receiver up tothe limiter tube may be similar to those described and claimed in the -aforesaid Foster application. Hence, such networks will be only generally described in this application.

' The tube I, of the 6SA7 type, is a pentagrid converter tube provided with a cathode 2, oscillator grid electrode 3, signal input electrode 4, a grounded suppressor grid 5, an output electrode 6 and positive screen grids 1 surrounding the signal grid 6. l

The collected signal energy is impressed upo a radio frequency amplifier which is tuned to the mid-band frequency. Since the complete frequency band is 42.6 to 43.4 mc., then the midband frequency of amplifier 8 is 43 mc. The signal collector can be a dipole, and atthe output terminals of amplifier 8 there will be present v in amplied form the various modulated carrier waves in the desired frequency band. By having the input circuit of the amplifier 8 of the bandpass type and covering the entire spectrum of frequencies desired, there will be developed across the signal input circuit coil 9 voltages of the various frequencies in the desired signal frequency band. Channel selection is accomplished by providing in shunt with coil 9 five independent channel selection coils. `Each of the coils l0 is shown with an arrow passing therethrough, and it is to be understood that the arrow denotes the fact that the inductance value of each of the coils is adjusted by the utilization of an adjustable brass cylinder, as disclosed in the aforesaid Foster application.

The grounded terminals of each of lcoils I0 is connected .to the low potential end of coil 9 through a direct current blocking condenser Il, while the opposite ends of each of coils I0 are connected tothe adjustable contact arms of each of a plurality of push-button switches I2. The

contact points of each of switches I2 are connected vto the signal grid lead, and the direct current blocking condenser I3 is insertedbetween the high potential terminal of coil 9 and the contact points o'f switches I2. It will be understood that the value ,of each of coils I is adjusted so that upon closing of a particular one of push-buttons I2 there will be selected from the band of impressed signal energy solely signal voltage of the channel frequency representative of .the particular push-button depressed.

'I'he tunable oscillator tank circuit is constructed in a manner similar to the signal input circuit just described. That is to say, the tank circuit coil I4 has connected vin shunt therewith a plurality of independent channel selection coils I5, and the latter coils are constructed in the same manner as explained in connection with coils II). 'Ihe push-button switches IB are connected so that closing of a particular push-button connects the corresponding coil I in shunt with the tank coil I4. The oscillator tank circuit frequency range is higher than the signal input frequency range, and is constantly different therefrom by the value of the operating I. F., which is chosen-to be 2.2 mc. 'I'he dotted line I1 is to be understood as symbolically representing the mechanical couplings between each corresponding pair of switches I2 and I6. That is to say, each of switches I2 has a corresponding tank circuit switch I6, and they are closed in pairs to effect channel selection. The channels themselves are arranged at intervals of 0.2 mc.

The oscillator grid 3 is connected to the high potential end of coil I4 through a condenser I8, while the resistor I9 connects the grid end of condenser I8 to ground. The low potential end of coil I4 is established at ground potential, and the cathode 2 of tube I is connected to an intermediate tap 2Il on the oscillator tank coil I4. There is developed across the I. F.tuned output circuit 2| the FM wave energy which has been selected at the input circuit, but the mid-channel frequency has the I. F. value of 2.2 mc. It is not believed necessary to described the manner in which I. F. voltages are developed by the converter tube, since those skilled in the art are fully aware of the manner of functioning of an electron-coupled converter of the type shown herein. 'I'he tube I and its associated circuits provide the well known form of electron-coupled combined local oscillator and first detector network.

The I. F. signal energy is transmitted through the bandpass network which comprises circuits 2| and 22 each flxedly tuned to the I. F. of 2.2 mc., it beingl pointed out that a resistor 23 is shunted across secondary circuit 22 in order to provide a wide band transmission network which is readily able to handle a wide band of I. F. energy. While the I. F. value may be generally chosen in the range of 2 to 4 mc., it is preferred to select an I. F. value of 2.2 mc., the band Width being of the order of 200 kilocycles (kc.). In general, the coupling between each of the I. F. transformer circuits is so adjusted that the desired band width is attained with the secondary damping resistor 23. The numeral 24 denotes an I. F. amplifier stage, and the tube employed may be of the same type as employed in stage 3, and an additional I. F. amplifier stage may follow tube 24 if desired. Each of these amplifier tubes may be, for example, of the 6AC7 type.

'Ihe amplified I. F. energy is then transmitted through the I. F. transformer 25 which feeds the input electrodes of the limiter tube. 'I'he transformer 25 has its primary and secondary circuits each fixedly resonated to the operating I. F. value, and the secondarydamping resistor 26 performs a function similar to that described in connection with ,resistor 23.y The numeral 2`I denotes the BAC? type limiter tube. By virtue of its associated circuits this tube functions as a combined amplitude modulation (AM) limiter and automatic volume control (AVC) stage. 'Ihe function of the limiter tube is essentially substantially to eliminate any amplitude modulation effect in the output thereof. That is to say, since the energy applied to the following FM detector should be a pure FM wave, it is necessary to subject the output of the I. F. amplifier to a limiting device in order to suppress any amplitude modulation which may have appeared on the envelope of the FM carrier wave. brief, then a limiter stage is a device whose output is constant for wide variations in the amplitude of the applied signal. However, this is necessary only because of the inherent limitations of the detector, and thus the limiter might properly be considered as an integral part of the detection circuit;

Considering, first, the electrical connections to the electrodes of the limiter tube, the cathode is established at ground potential, while the low potential end of the input circuit 25 is connected to the grounded cathode through a resistor 3| shunted by condenser 32. 'I'he signal input electrode 28 of the limiter tube is connected to the high potential end of the input circuit. Across resistor 3| is developed a direct current voltage which is derived from the grid current iiow of the limiter tube. The direct current voltage is utilized as a bias for grid 28, and for a function to be later described. 'I'he voltage is, furthermore, employed for securing AVC action, and for this purpose there is shunted across resistor 3| a second resistor 33, an intermediate point thereof being connected to the lead 34 designated as AVC.

Those skilled in the art are fully aware of the manner of utilizing an AVC circuit, and it is only necessary to point out that lead 34 applies the AVC bias to the signal grids of each of amplifiers 8 and 24 through filter resistors 35. 'I'he applied AVC bias acts to increase the effective signal grid bias of each of the controlled ampliiiers so as to reduce the gain of each of the controlled amplifiers. In this way it is possible to maintain the selected mid-channel frequency amplitude at the input circuit 25 at a desired amplitude. As mentioned in the aforesaid Foster application each of the controlled amplifiers preferably includes a degenerating resistor in its cathode circuit in order to prevent detuning effects on the controlled amplifier input circuits due to the variable signal grid biasing action of A. the AVC circuit.

The screen grid 29 of tube 2I is connected to the source of positive voltage which also supplies the plate 30 through a resistor 36, the screen grid, furthermore, being connected to ground through a second resistor 31, the screen grid end of the resistor 31 being bypassed by a condenser 38. The plate 30 is connected to the positive potential source through a resistor 33 whose plate end is, also, by-passed to ground through a condenser 40. Across the I. F.tuned output circuit 4| there is developed I. F. energy practically free of amplitude modulation on its modulation envelope. A damping resistor may be shunted across the circuit 4I in order to impart the desired wide band characteristic thereto. The AM limiter operates with grid currentdeveloped bias and with reduced screen potential to secure limiting action on both the positive and l negative halves-of the wave envelope. The limiting on the positive half of the wave envelope is due to the developed grid bias fluctuations, and on the negativerhalf of the wave the limiting action is secured by virtue of plate current cuto. The action of the limiter and the -reasons for the choice of circuit constants can better be un derstood by reference to Figs. 5A and 5B.

Resistor 3| and condenser 32 determine the time constant on discharge. Resistor 26 is the secondary circuit damping resistor. Resistors 31 and 36 comprise the bleeder for fixing the screen potential, and hence the plate current characteristic. The time constant for charging condenser 32 is determined by the value of the condenser and the value of the charging resistance. The equivalent resistance on the charge portion of the cycle is d ue to resistor 26, the I. F. transformer impedance, and the limiter tube.

If the discharge time constant of 3|32 is too long, amplitude variations in the positive direction will not be limited, as is graphically shown in Fig. 5A. This gure Arepresents a condition when the discharge time constant is appreciably longer than the charge time constant, and is long in comparison with the time of amplitude'variation cycle. 'Ihe signal on the limiter input grid is seen to consist of a carrier with amplitude modulation imposed thereon. Under these conditions the bias developed across the resistor 3| will be proportional to the peak signal, and only the tips of the applied wave draw grid current. The negative half of the envelope is seen to swing beyond plate current cut-olf so that amplitude variations in that direction do not appear in the output. However, amplitude variations in the positive direction do appear in the output signal.

The condition when charge and discharge time constants are made sufficiently short to follow variations in amplitude at modulation rate. but are still long compared to the time of a carrier frequency cycle, are illustrated in Fig. 5B. Under such conditions the bias, developed by the grid current, changes during the amplitude variation cycle. The grid-of the limiter tube acts like a diode, and if it were an eilicient diode only enough grid current would be drawn from the charging source to supply the-necessary bias, and the positive peaks of the wave would be lined up along the zero grid potential axis. Due to the fact that no diode is perfect, and the fact that the charge source has some resistance, some variation in positive direction still exists, but much reduced over the condition shown in Fig. 5A.

Under the conditions of Fig. 5B, when. the signal is suiciently large to swing in a negative direction beyond plate current ut-oi, the limiting is obtained on both halves of the wave envelope. The charge time constant is usually sufficiently small, but attention must be paid to the value of the discharge resistance 3| and the capacitor 32 so that their product is not` too large to follow amplitude fluctuations of the highest frequency it is desired to limit.l For example, if it is desired to limit amplitude modulation iiuctuations up to 10,000 cycles per second,

tion of input voltage at the signal collector.

then network 3 I-32 should be of the order of the time of V4 cycle of that wave, or 25A micro-seconds. This value is still large compared with the time of the 2 mc. intermediate frequency cycle, in other words, 0.5 micro-seconds, so it will not vary at. the carrier rate. It is for this reason that there should be employed limiter discharge time constants of the order of the 1 to 25 microseconds. y

If applied I. F. signal voltage is very large compared to the bias for plate current-cut-off, current will be drawn only on the tips of the carrier wave. -The tube is then `operating as a class C amplifier, and-in addition to the energy at the I. F., harmonics thereof will be generated. The LF. transformer in the output circuit of the limiter tube can pass only the fundamental component so that the signal energy delivered to the detector will then decrease. This effect is evidenced by a limiter characteristic which reaches a maximum output as the signal input is in'- creased from zero, and then the output decreases with further increase in signal. One way of preventing this decrease in limiter output with large signals is to apply AVC to some of the tubes preceding the limiter tube. The use of AVC will prevent a falling limiter characteristic, and, in addition, if applied to the radio frequency ampliiler 3 will prevent generation of ultra-high frequency harmonics through over-shooting of the signal grid. bias of the converter tube. The 43 mc. bandis suillciently narrow so that the harmonicsv of the oscillator and signal just fail to fall within the I. F. value, but it is conceivable that some of the stationsadjacent to that band may cause difculty in the absence of AVC. By tapping ofi for AVC action a substantially less amount of the direct 'current voltage developed across resistor 3|, it is possible to secure maximum benet from the AVC action. If all the AVC bias were used the point at which limiting starts would occur at a higher input level.

In Fig. 2 there is shown graphically the characteristics of the limiter stage. The upper curve shows the.. relation between the output of the limiter asa function of the input voltage on the signalv collector of the receiving system, as measured in the detector stage with a signal tuned approximately kc. oi the center frequency. This curve indicates that the full output of the limiter is developed with about 7 micro-volts input. The lower curve shows the peak voltage developed across the limiter grid bias resistor 3| as a func- By virtue of the. tapped resistor 33 in shunt with resistor 3|, about 23 percent of the voltage is used as the AVC bias. The bias is proportional to the signal. At no-signal state no bias (other than the small amount due to contact potential andy e signal increases bias still further, but thatportion of signal swing beyond cut-oi is not reproduced in the output, thereby giving limiting action for negative grid swing direction. The value of cut-oil bias is dependent on screen voltage; the lower the screen voltage the lower the bias which will produce plate current cut-oil. However, the screen voltage cannot be made -too low or the initial electron velocity j comes into play. practical lower limit is some 20 or 30 volts. I'he value of screen potential sets the output of the frequency of the order of 2.1 mc.

limiter, and hence of the detector for a given frequency deviation. 'As the screen voltage is raised more signal on the grid is required, and hence more output results before limiting action takes place.

Resistors 36-31 draw current which is high compared to the current drawn by the screen so that as the grid bias is changed by the applied signal the screen potential does not change. The screen potential exercises the predominant effect in determining the bias at which plate current cut-off occurs. The plate potential is determined by the connection of resistor 39 to the B plus voltage terminal and by the voltage drop due to plate current flow through resistor 39. With no signal input the plate potential is of the same order as the screen potential. It is desirable to use and unvariable screen potential so that the plate current cut-oil characteristic of thev tube does not change with magnitude of applied signal.

'I'he AVC action tends to hold the signal voltage applied to the limiter constant as the signal varies, but cannot completelyaccomplish this purpose because the decreased gain of the amplifier tubes to which AVC is applied results from increased bias, and some increase in signal is necessary to provide this increased bias. The output of the limiter is prevented from rising despite the increased signal on the negative excursions of grid potential by plate current cut-off, but may rise in the positive direction because of the nite grid impedance of the tube. That is, the positive excursions of the signal'extend into the positive grid region. If the plate potential is maintained at a high fixed potential the limiter output rises with increasing signal input, because, as has been pointed out, AVC action cannot be perfect. If the plate potential be maintained at a low fixed value, on the other hand, the limiter output will decrease with increasing signal above the value of input which causes plate voltage overload. If, now, the plate is fed through a dropping resistor, as shown, the potential of the plate rises with signal increase since the higher developed grid bias reduces the alternating plate current, and, hence, the voltage drop through resistor 39. By proper choice of the value of resistor 39 the plate potential may be made to increase in proper proportion to increased signal so that the output from the limiter is constant for any value of signal above that necessary to initiate plate current cut-oil. The combination of partial AVC; fixed screen potential/and plate potential variable with signal is thus seen to provide a more constant limiter output than can be obtained without this combination of effects.

The purely FM signal energy lat circuit ll is applied to the input circuits of the double diode tube 50, which may be of the GHG type. The tube has two pair of independent diode electrodes therein. Anode 5I and cathode 52 comprise onediode, while anode 53 and cathode 54 provide the second diode. The cathodes 52 and 54 are connected by a pair of resistors 51 and 58.

The junction of the resistor 5B and cathode 54 is at ground potential, and independent I. F. carrier bypass condensers 55 and 56 shunt each of the resistors. The diode 5l-52 has an input circuit comprising coil 59 and shunt condenser 60, and the input circuit is xedly tuned to a The auxiliary coil 6| is arranged in series with the coil 59 and condenser 90, and coil 59 is magnetically coupied to circuit al.

Each of circuits'BQ-IU and 63-64 are tuned by equal frequency amounts to opposite sides of the I. F. value. For example, the circuit 59-89 may be iixedly resonated to 2.1 mc., while the other input circuit is tuned to 2.3 mc. Hence, there will be developed across each of resistors 51 and 58 rectified signal voltage. The rectifiers being in opposed relation, there will be tapped oi at the cathode end of resistor 51 the difference lof the voltages across the two load resistors.l

Switch 10, when closed on the lower contact provides a path for tapping off the audio voltage developed across the 'resistive load of the rectiflers. The resistor-condenser path 1I-12, shunted across the detector output, provides a de-emphasizing network for the high frequency components of the modulation voltage output. Since FM transmitters usually employ some high audio frequency component emphasis, the path 1|-12 will provide a compensation therefor; this action arises by Virtue of the attenuation of the high audio components. The audio voltage across condenser 12 is then utilized by any desired form of audio utilization network, and the usual tone-compensated audio volume control device 13 may be connected across con-I denser 12.

Since the detector stage comprises a pair of oppositely mistuned, relative to I. F. value, rectiers having output load resistors connected in phase opposition, or back-to-back, the detector characteristic will be a slope between the offresonance frequencies of the rectiiiers. The output of the detector will depend on frequency variation, and not on amplitude variation. In an FM wave the instantaneous frequency is varied at an audio rate, so that if such a wave is applied to the detector shown the output will vary at the audio rate, and, hence, reconstitute the original modulation imposed on the transmitter. The magnitude of the audio frequency output is proportional to the slope of the detector characteristic and to the amount of frequency deviation. The slope of the characteristic is dependent on circuit constants. Hence, in

a given detector, the output is proportional to frequency deviation; that is, to the amplitude of the audio frequency wave modulating the transmitter.

Despite the fact that off-resonance circuits are used, alignment of the two detector input circuits may be performed at the center I. F. value. The auxiliary coils 6I and 65 function as the aligning coils. During alignment connection is made to the upper contact by switch arm 66; that is, coil 6I is short circuited. In thisposition of switch 66 the circuits 4I, 59-69, 63-64 are each tuned to the I. F. value of 2.2 mc. The switch arm 66 is then shifted to the lower contact to short circuit coil 65. This results in a decrease of the resonant frequency of circuit 59-60, and a concurrent increase of the resonant frequency of circuit 53-64 by a like frequency amount. The coils 59 and 63 may have the same magnitudes, but coils 6I and 65 are of different values.

In Fig. 3 there is shown the FM detector characteristic. -The separation between the peaks of the curve depends upon the ratio of the inductance of coils 6I and B5 to that of coils 59 and 88. a Inductances Si and 85 may conveniently be outside the I. F. shield can so that its value may be readily varied to obtain the desired separation If an FM wave is one with unvarying amplitude whose frequency is cyclically altered above and below its mean unmodulated value (l. FJ, then Fig. 3 shows how each rectier supplies the variable unidirectional voltage from its load resistor. The algebraic sum of the two voltages, varying at an audio rate, is supplied to the audio circuit when switch is closed on the lower contact. If, however, switch 10 is closed on the upper contact, the grid .circuit of the limiter tube functions as a diode rectifier, and provides audio voltage over lead 80 to the audio circuit. The latter is the A. M. reception position of switch 10. The detector now acts solely as an AFC discriminator in the usual manner.

As explained in the aforesaid Foster application, it is highly necessary to apply AFC to the local oscillator because of the use of push-button tuning, and the fact that at the ultra-high frequencies employed slight departures from correct oscillator frequencies will result in considerable mistuning and consequent severe distortion. The tolerance in frequency drift is materially less in these high frequencies. Further, since it is essential to have the applied I. F. energy have a mid-channel frequency located at the center between the two peaks of the characteristic of Fig. 3, AFC becomes practically lessential for ease of tuning operation.

The FM detector obviously is inherently an AFC discriminator; Fig. 3 illustrates this fact.

. Those skilled in the art are fully aware of the manner of employing AFC in a superheterodyne receiver. Reference is made to application Serial No. 130,630 filed March 13, 1937, of D. E. Foster to show such an AFC system. In the present application, a highly desirable and improved form of frequency control tube circuit is utilized across the oscillator tank coil i4. The control tube 8| is a high transconductance tube of the 6AC7 type, and its plate 82 is connected to the high potential side of coil |4 by the direct current blocking condenser 83. The plate is connected to a source of positive potential through a choke coil 84. The cathode of tube 8| is connected to ground through self-bias resistor 85 shunted by bypass condenser 86.- There is applied to grid 81 an alternating voltage which is in quadrature with the plate alternating potential. The grid 81 is connected to point 20 on coil |4 for this purpose through a path including direct current blocking condenser 88 and resistor 89.

The oscillator tank circuit current flowing through the path including resistor 89, condenser 88 and the grid to cathode capacity 90 (shown dotted) develops across capacity 90 the quadrature voltage. As is knownand as explained in the aforesaid Foster AFC application, between the plate l82 and ground is 4simulated an inductive effect which appears in shunt to coil I4. The dotted coll 9| is the simulated inductive eiiect 9| depends on the gain of tube 8|.

due to tube 8 i The magnitude of the inductance Hence, the signal grid 81 is connected to ground through a source of frequency-dependent direct current voltage. The latter source is the direct current voltage developed across resistive load 51-f58. The audio pulsations of the voltage across the latter are ltered out by lter resistor 92 and condenser 93. 'I'he lead 94, designated as the AFC lead, connects resistor 92 to grid 81 through filter resistors 95 and 96, theV junction of the latter being bypassed yto ground by conp denser S1.

- The characteristic of the frequency control tube circuit is snown in Fig. 4. There are plotted Control tubebias volts against Oscillator frequency changeV (kc.).` It will be noted that a satisfactorily wide range of correction can be secured with the circuit. This characteristic together with that of the detector (Fig. 3) determine the mistuning correction of the AFC circuit for signals above the limiter stage threshold. This control action is seen to be approximately 10 to 1 so that mistuning of the oscillator tank circuit of 100 kc. would cause a mistuning from the I. F.'value of only 10 kc. It will be clear to those skilled in the art that if the I. F. energy has a center frequency which departs from the assigned 2.2 mc. value, that rectier whose input circuit is closer to the shiftedvalue will dominate in direct current voltage production. The resulting bias applied over AFC lead 94 will vary the gain of tube 8| in that sense, and to an extent, such that the effect 9| will vary to cause the oscillation frequency to be shifted so as to compensate for the I. F. center `frequency shift. It is pointed out that the inductance 9| increases in value as the gain of the control tube is decreased by AFC bias, and that an increase in shunt inductance results in a decreased tank frequency.

With respect to the simulated inductive effect 9|, it can be shown that its value is:

, RC Lm In the aforesaidA formula, C is. the capacity'su R is the series resistor; Gm is the mutual conductance of the control tube; and K is the portion of the tank voltage appearing between tap 20 and ground. Since K is less than unity this would appear to give ua higher L; therefore, less shift than if across the entire coil. However, one may reduce R in proportion to K and obtain the same voltage across C as if the entire coil voltage were used; Suppose K were 0.3 then if R is reduced by some factor from the value that would be used across the entire coil, the same radio frequency voltage is applied to the control tube grid. But if K=0.3 the inductance across which it is tapped becomes 0.09 of total inductance (inductance being approximately proportional to the square of the number of turns) so that the shunting effect of R and of input resistance of tube is decreased by a factor of 0.3 thereby giving more nearly pure quadrature voltage, and better control action.

The grid 81 of tube 8| is tapped down to 20 for the following reason. All amplier tubes that might be used as tube 8| have high input conductance (low resistance) at the ultra-high frequencies, hence they seriously load any tuned circuit if connected across the" entire tuned circuit. By tapping the grid down this loading eiect is reduced. The grid circuit of grid 81 is tapped down to the oscillator cathode tap. However, this is not necessarily the only place that it could be tapped. It could be tapped either above or below the cathode tap, and, further, the oscillator circuit might not use a hot" cathode but might obtain feedback coupling by mutual inductance coupling'. In the oase .of an oscillator using mutual inductance coupling the advantage of tapping the grid circuit of grid 81 across part of the coil would still prevail at ultra-high frequencies.

The advantages of the present frequency'control tube circuit will be seen to involve thefollowing. Firstly, the inherent capacity 90 can be employed as a quadrature condenser. Second, the grid tap on coil Il prevents the low input resistance of tube 8l from damping the tank circuit. Thirdly, the high transconductance tube Il gives sufficient control with the small inductances I 5 required for the ultra-high frequency range.

'I'he receiver shown herein will readily receive FM transmissions using deviations of about '75 kc. and transmitting modulation frequencies up to 15,000 cycles. If the deviation is less a proportionate reduction in I. F. band width should be made, and if less than 15,000 cycle modulation is employed then the audio pass band should be likewise reduced to maintain noise reducing capabilities. output with 12 kc. deviation for any input over 3 microvolts, and it will be sensitive. The noise threshold for uctuation noise is at about 3 microvolts so that noise reduction is obtained above that input. Since the limiter operates fully at 6 microvolts above the latter input the receiver will be completely quiet as far as fluctuation noise is concerned.

It is pointed out that the present system is adapted for receiving phase modulated carrier waves. Since the electrical relations between phase and frequency modulation are similar, it is obvious that a common receiving system can be used for receiving both types of waves. The term timing modulation is used herein to describe a common characteristic of phase and frequency modulated waves, since in both cases the modulation varies the timing of the carrier wave frequency with respect to a hypothetical point. The expression timing modulated carrier waves used in the claims, accordingly. is to be understood as a generic expression covering either phase or frequency modulated carrier waves.

Since in phase modulation the frequency deviation is proportional to the modulating fre- The receiver will deliver 0.5 watt V:

quency, uniform output for all audio frequencies when the receiver herein described is supplied with phase modulated waves may be secured by appropriate values of resistor 41| and capacitor 12.. For example, if resistor 1I be made high in comparison with the reactance of '72 for even low audio frequencies (say down to cycles) the voltage across 12 will be inversely proportional to audio frequency, and uniform frequency response will be secured from phase-modulated waves.

'I'he following. circuit constants are given, it being clearly understood however that they are in no way restrictive, but are merely illustrative:

Ras=2000 ohms Rue-.50,000 ohms Ras=50,000 ohms Ras=500 ohms R'zi=20,000 Ohms Rns= 1 megohm Rm=250,000 ohms Ra=250,000 ohms Ran= 100,000 ohms R3c=10,'000 ohms Ra1=9,000 Ohms Raa=1 megohm+300,000 ohms R3x=50,000 Ohms Rn=25,000 ohms Lu=40 microhenries L4i=56 microhenries Ln=56 microhenries Lsa=56 microhenries Lsi=5.6 microhenries Lss=5.0 microhenries Caz=50 micromicrofarads (mmf.) C::'=0.1 microfarad (mf.) C4o=5000 mmf.

C5s=50 mmf.

Cu=50 mmf.

C1a= 1000 mmf.

C03=0.1 mf.

While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

1. In a frequency modulation receiver, a tube having input and output electrodes, a frequency modulated wave input circuit, an impedance network connected between said frequency modulated wave input circuit and said tube input electrodes, an output circuit connected to the output electrodes, said impedance network deriving a direct current voltage from the frequency modulated wave applied thereto, the direct current voltage so derived serving to bias the input electrode, and the time constant of said impedance network being of the order of 1 to 25 microseconds.

2. In a frequency modulation receiver, a tube z having input and outputelectrodes, a frequency modulated wave input circuit, an impedance network connected between said. frequency modulated wave input circuit and said tube input electrodes, an output circuit connected to the output electrodes. said impedance network deriving a direct current voltage from the frequency modulated wave applied thereto, the direct current voltage so derived serving to bias the input electrode, the time constant of said impedance network being of the order of 1 to 25 microseconds, said tube including a positive screen grid, and resistive means in circuit with the screen grid to maintain it at an invariable direct current potential.

3. In a frequency modulation receiver, a tube having input and output electrodes, a frequency modulated wave input circuit, an impedance network connected between said frequency modu lated wave input circuit and said tube input electrodes, an output circuit connected to the output electrodes, said impedance network deriving a direct current voltage from the frequency modulated wave applied thereto, the direct current voltage so derived serving to bias the input electrode, and the time constant of said impedance network being of the order of 1 to 25 microseconds, said tube including a posiwave is applied in increasing amplitude to the input electrode.

4. In a frequency modulation receiver, a tube having input and output electrodes, a frequency modulated wave input circuit, an impedance network connected between said frequency modulated wave input circuit and said tube input electrodes, an output circuit connected to the output electrodes, said impedance network deriving a direct current voltage from the frequency modulated wave applied thereto, the direct current voltageso derived serving to bias the input electrode, and the time constant of said impedance network being of the order of 1 to 25 microseconds, and means connected to said impedance network for applying at least a portion of said voltage to a point of said receiver preceding said input circuit thereby to control the amplitude of the modulated waves at said input circuit.

5. In combination, in an amplitude limiter network for timing modulated carrier waves, a tube provided with at least a cathode, wave input electrode and an output electrode, a wave input circuit upon which said waves are impressed cou-v ment and resistor element having charge and discharge time constants which are suiiiciently short to follow variations in amplitude at modulation rate, but which are still long compared to the time of a carrier frequency cycle.

6. In combination, in an amplitude limiter network, for timing modulated carrier waves. a tube provided with atleast a cathode, wave input electrode and an output electrode, a wave input circuit upon which said waves are impressed coupled to the cathode and input electrode, a resistor element in circuit with said input electrode and cathode for developing'thereacross a unidirectional voltage from current iiowing between the cathode and input electrode, and a reactive element operatively associated with the resistor element, said reactive element and resistor element havingV a time constant of the order of 1 to micro-seconds, and resistive means in circuit with the output electrode for permitting the direct current potential thereof to vary inY response to variations in the input electrode direct current potential.

'7. In a system for transmitting timing modulated carrier waves, a network for limiting amplitude modulation eiects in said waves, said network comprising a tube provided with at least an electron emission element, an input lelectrode and an output electrode, a modulated carrier wave input circuit connected between said input electrode and emission element, an impedance network connected between said emission element and input electrode for developing a unidirectional voltage from current owing between said element and input electrode, said voltage being applied to said input electrode as a bias therefor, and resistive means in circuit with said output electrode for permitting the direct current potential thereof to rise as said modulated waves are applied to said input circuit in increasing amplitude.

8. In a system for transmitting timing modulated carrier waves, a network for limiting amplitude modulation 'eects in said waves, said network comprising a tube provided with at least an electron emission element, an input electrode, a positive screen grid, and an output electrode, a modulated carrier wave input circuit connected between said input electrode and emission element, an impedance network connected between said emission element and input electrode for developing a unidirectional voltage from current flowing between said element and input electrode, said voltage being applied to said input electrode as a bias therefor, resistive means in circuit with said output electrode for permitting the direct current potential thereof to rise as said modulated waves are applied to said input circuit in increasing amplitude' and resistive means in circuit with said screen grid to maintain it at an invariable direct current potential.

9. In a system for transmitting timing modulated carrier waves, a network for limiting amplitude modulation effects in said waves, said network comprising a tube provided with at least an electron emission element, an input electrode, a positive screen grid, and an output electrode, a modulated carrier wave input circuit connected between said input electrode and emission element, an impedance network connected between said emission elementv and input electrode for developing a unidirectional voltage from current owing between said element and input electrode, said voltage being applied to said input electrode as al bias therefor, resistive means in circuit with said output electrode for permitting the direct current potential thereof to rise as said modulated waves are applied tov said input circuit in increasing amplitude, resistive means in circuit with said screen grid to maintain it at an invariable direct current potential, and means for quency transmission network and detection network of a superheterodyne receiver of timing modulated carrier waves, a limiter network for` minimizing amplitude modulation effects in said waves, said limiter network comprising a tube having at least a cathode, control grid and output electrode, a wave input circuit coupled to said transmission network and connected to said grid and cathode, means coupling said output electrode to said detection network, a resistor in circuit with the grid and cathode to developzdirect current voltage from grid current flow whereby said voltage serves as a bias for the grid, a condenser in shunt with the resistor, said resistor and condenser having a time constant which is suiiiciently short substantially to reduce amplitude variations in the limiter output electrode circuit, and means in circuit with said output electrode to permit variation of the output electrode direct current potential with modulated carrier wave amplitude variation.

11. In combination with the'intermediate frequency transmission network and detection network of a superheterodyne receiver of timing modulated carrier waves, a limiter network for minimizing amplitude modulation effects in said waves, said limiter network comprising a tube having at least a cathode, control grid and output electrode, a wave input circuit coupled to said transmission network and connected to said grid and cathode, means coupling said output electrode to said detection network, a resistor in circuit with the grid and cathode to develop direct current voltage from grid current flow whereby said voltage serves as a bias for the grid, a condenser in shunt with the resistor, said resistor and condenser having a time constant.

which is sumcientiy shortsubatantiaily to reduce amplitude variations in the limiter output electrode circuit, means in circuit with said output electrode to permit variation of the output elec- 10 

